Unsubscribe link was "blocked"

I had a contact try to unsubscribe from the link at the bottom of our emails in a campaign, but receive the message that the link was “blocked”

What is the problem causing this and how can we fix it?

That doesn’t sound like an IS message. Almost sounds like a setting on the client side but it’s hard to say without seeing it ourselves. Did you try to view the email sent to the client and click the unsubscribe link yourself?